In a power module which uses a semiconductor device, such as an IGBT (Insulated Gate Bipolar Transistor), the semiconductor device must be held at a predetermined temperature or lower by means of efficiently radiating heat generated therefrom. Conventionally, in order to meet the requirement, a heat radiator is used. The heat radiator includes an insulating substrate which is formed of a ceramic, such as aluminum oxide (Al2O3) or aluminum nitride (AlN), and whose first side serves as a heat-generating-element-mounting side, and a heat sink which is formed of a high-thermal-conduction metal, such as aluminum or copper (including copper alloys; hereinafter, the same is applied), and is soldered to a second side of the insulating substrate. A semiconductor device is soldered to the heat-generating-element-mounting side of the insulating substrate of the heat radiator, thereby forming the power module.
A power module used in, for example, a hybrid car must maintain the heat radiation performance of a heat radiator over a long term. The above-mentioned conventional heat radiator involves the following problem. Under some working conditions, thermal stress arises from a difference in coefficient of linear thermal expansion between the insulating substrate and the heat sink and causes cracking in the insulating substrate, cracking in a solder layer which bonds the insulating substrate and the heat sink together, or warpage of a bond surface of the heat sink bonded to the insulating substrate. Such cracking or warpage impairs heat radiation performance.
A proposed heat radiator in which the above problem is solved includes an insulating substrate whose first side serves as a heat-generating-element-mounting side, a heat radiation member which is soldered to a second side of the insulating substrate, and a heat sink which is screwed on the heat radiation member. The heat radiation member includes a pair of platelike heat-radiation-member bodies formed of a high-thermal-conduction material, such as aluminum or copper, and a low-thermal-expansion material, such as an Invar alloy, intervening between the platelike heat-radiation-member bodies. (Refer to Patent Document 1).
However, the heat radiator described in Patent Document 1 must use the heat radiation member formed of a high-thermal-conduction material and a low-thermal-expansion material; thus, material cost is increased. Furthermore, since the heat radiation member and the heat sink are merely screwed together, thermal conduction therebetween is insufficient, resulting in a failure to provide sufficient heat radiation performance.    Patent Document 1: Japanese Patent Application Laid-Open (kokai) No. 2004-153075